 Welcome to Physiology Open. In this video we will see what are ECG waves, intervals and segments, how to remember duration of various intervals and segments and even ECG waves and how to read an ECG. ECG that is the electrocardiography is recording a spread of electrical impulse in heart and how this electrical impulse is recorded as various waves, intervals, segments in ECG. So to understand these let's see a lead to ECG. So here this picture is showing a lead to ECG. So this is the thermal paper over which ECG is recorded. This portion is showing the calibration that is the voltage recorded as some distance on a paper. The standard calibration is that one millivolt is recorded as one centimeter on a thermal paper. This portion is the recording of the spread of the electrical impulse in the heart. So here it is showing two electrical impulse recording. So let's focus on recording of one electrical activity here. The first wave which is seen here is P wave. It occurs due to atrial depolarization. After atrial depolarization there is some time during which atria depolarized and ventricular is still at resting membrane potential because there is a pause at AV node. So no potential is recorded during this time. Then we get another wave or we can say a wave complex known as QRS complex. The QRS complex represents ventricular depolarization. Then there is some time in between when again no voltage is recorded that is the time in between the ventricular depolarization and ventricular repolarization. After that we get another wave that is the T wave which is occurring due to ventricular repolarization. Now you may ask where is atrial repolarization recorded? Atrial repolarization occurs at the same time as that of ventricular depolarization so that the voltage change there is merged with QRS complex. Now this is a convention which we follow while naming the waves that is starting with P wave till T wave especially you should focus on QRS complex. QRS complex represents ventricular depolarization right? Now how is it named? The first negative wave that is recorded during ventricular depolarization is known as a Q wave. First positive wave is known as R wave and the negative wave which occurs after the first positive wave is S wave. But why we need to know all this? Because there may be some leads in which you don't see first negative Q wave directly there may be a positive wave. Then that wave we don't call as Q wave we call it as R wave because by convention first positive wave is known as R wave but whole complex we call as QRS complex only. Second as we will see later in the video that we have something known as PR segments and PR intervals not PQ segment and PQ intervals but even though they are known as PR segment and PR interval we mark them till the beginning of QRS complex. Okay so we have dealt with waves of ECG here now let us see various intervals and segments. First thing to remember is segments do not have any waves between them but intervals include waves. So what are these segments and intervals? We will talk about three segments and five intervals. The segments are PR segment, ST segment, TP segment. So PR segment remember segment do not include waves so PR segment is from the end of P wave that is this point to the beginning of QRS complex that is this point. So this much is PR segment. So see here even though we are saying PR segment we are marking till the beginning of QRS complex. So PR segment represents AV nodal pause. During this time atria is depolarized but ventricles are a trusting membrane potential. ST segment is from the end of QRS complex that is this point to the beginning of T wave. So till here so this is ST segment. See again that there is no wave in between. During ST segment ventricles are depolarized and no current is flowing through the heart. Now TP segment is from end of T wave of one record to P wave of record of another impulse. So this portion is TP segment. Now let us see the intervals. First is PR interval. PR interval is from beginning of P wave to beginning of QRS complex. As already told the intervals includes the waves. So PR interval is from beginning of P wave. It includes the P wave to the beginning of the QRS complex. Next is ST interval. ST interval is from end of QRS complex to end of T wave. So it is including the T wave. This is ST interval. While QT interval is from beginning of QRS complex to end of T wave. So that means from here beginning of QRS complex to end of T wave. Now sometimes it becomes very confusing to remember these intervals from where they begin and where they end. So I remember it by one stupid mnemonic. I don't know it may help you also. As already told intervals include waves while segments do not include waves. So which waves do these intervals include? See PR interval is this side of ECG. So it includes this side of wave that is P wave. Now ST interval is this side of ECG. So include this side wave that is T wave. While QT interval includes both the waves. See it includes from the beginning of the QRS complex to end of T wave. That means it includes both QRS complex and T wave. Now there are two other intervals which are of significance for us. One is RR interval taken from the peak of one R wave to the peak of another R wave. So this is RR interval. While in certain abnormalities we also need PP interval. That is from the peak of P wave to peak of another P wave. So when we will talk about how to read ECG we will tell that where this PP interval is needed. Now let us see what are the normal durations of various wave segments and intervals. For that we'll just see one concept and it will help you remember the various durations. See as signal starts from SA node it takes 0.03 seconds to reach AV node and further 0.06 seconds to depolarize the atria. So total time for atrial depolarization we are just adding this 0.06 and 0.03. It is 0.09 seconds. Since P wave is representing atrial depolarization so this P wave duration is 0.09 seconds. Now impulse stops at AV node for 0.1 second that is the AV nodal delay and then only the ventricular depolarization starts. So if we add this the traveling time 0.03 seconds and this 0.1 second it comes to 0.13 seconds. That means from the beginning of the atrial depolarization to the beginning of the ventricular depolarization it is 0.13 seconds. This is what? This is nothing but PR interval. Then ventricular depolarization it takes around 0.03 seconds to occur from year to year and then further spread by Purkinje fibers also takes 0.03 seconds. So again it is 0.06 seconds. So this QRS complex see it represents a ventricular depolarization and we are seeing that ventricular depolarization is occurring in 0.06 seconds. Simple to remember right? You have to remember only this 0.03 everywhere and AV nodal delay as 0.1 seconds and you can easily tell how much is the duration of these waves and intervals. Now to calculate PR segment you just minus PR interval from P wave and you will get the duration of the PR segment that is 0.04 second. So basically you need to remember only two more values that is the ST segment and the duration of the T wave. So ST segment duration is 0.08 to 0.12 second and duration of the T wave is 0.12 to 0.2 seconds. So when you know the duration of the ST segment and T wave then you can easily calculate what will be the duration of the ST interval right? That is from the end of the QRS complex till the end of the T wave is your ST interval. Just add the two durations and you will get the duration of the ST interval. Similarly you can get also the duration of the QT interval.